The present invention relates to a process for desalination of saline solution. The present invention also relates to an apparatus for carrying out this process, and the use of said process or apparatus for the reduction of the volume of concentrated saline solution byproduct of a reverse osmosis membrane desalination plant, or in a device or plant or process for producing desalinated water, for salt production, for co-production of power and desalinated water, or for air conditioning.
In the present application, the term “saline solution” refers to any aqueous solution containing at least one dissolved salt, and the term “first byproduct stream” refers to a concentrated saline solution byproduct obtained from a reverse osmosis (RO) membrane desalination plant. Other examples of concentrated saline solutions include seawater, brackish water, or mining water. It is noted that the saline solution feed stream and the first byproduct stream of a RO membrane desalination plant, as well as the other above examples of concentrated saline solutions, all contain at least one dissolved inorganic salt, typically NaCl.
Fresh water is required for public and domestic water consumption, in farming for irrigation and livestock, and industrially for a variety of processes. In the present application, the term “fresh water” generally refers to water characterized by having low concentrations of dissolved salts and other total dissolved solids and specifically excludes seawater and brackish water. In one embodiment, “fresh water” refers to water containing less than 3,000, preferably less than 1,000, most preferably less than 500 ppm dissolved salts. Drinking water is an example of fresh water. Due to a lack of uniform distribution of fresh water supplies, it is necessary in many regions to obtain fresh water by desalination of a saline solution, for example from the sea. In the present application, the term “first product water stream” refers to the fresh water obtained by the process, apparatus or use of the invention.
Reverse osmosis (RO) is the most widespread technology for desalination of water, and a reverse osmosis desalination apparatus and method are disclosed, for example, in U.S. Pat. No. 4,115,274 or U.S. Pat. No. 4,125,463. It is a membrane separation process in which water is recovered from a saline solution by pressurizing the solution beyond its osmotic pressure and essentially using the membrane to filter out the salt ions from the pressurized solution and allow only the water to pass. The main energy consumption in RO technology results from the pressurization of the saline solution. Relatively low energy consumption with RO technology can be advantageously obtained when the energy losses resulting from releasing the pressure of the concentrated saline solution are minimized by using devices to recover the mechanical compressive energy from the discharged concentrated saline solution stream (first byproduct stream). RO technology is the most widespread commercial technology for desalination due to its favorable economics largely resulting from relatively favorable energy consumption.
Nonetheless RO technology has its disadvantages. Since the pressure required to recover additional fresh water increases as the saline solution or brine stream is concentrated, the water recovery rate of RO systems tends to be low. A related further major disadvantage is then the cost and environmental impact of disposing of the large volume streams of concentrated saline solution byproduct (first byproduct streams) from the RO plant, particularly for inland RO plants. For example, the saline solution byproduct is often discharged to the sea or inland surface water or injected into deep wells. Such practices are not environmentally friendly, and thus they are no longer acceptable. Therefore it would be desirable to have a process and an apparatus for increasing the volume of fresh water recovered, reducing the volume of the first byproduct stream (concentrated saline solution) from RO plants, and without inducing additional harm to the environment.
Thermally-based concentration methods to reduce the volume of first byproduct streams of RO systems are known, such as the flash evaporation method disclosed in U.S. Pat. No. 4,083,781, the forced evaporation method disclosed in U.S. Pat. No. 4,434,057; and the combustion heat evaporation method of U.S. Pat. No. 5,695,643. Such thermally-based concentration methods have the disadvantage of being energy intensive and thus costly. In addition, they are susceptible to scale formation and its associated thermal and mechanical problems. Alternatively, first byproduct streams may be concentrated by solar ponds having low energy costs, but this thermal method requires large amounts of land and direct sunlight and suffers from low productivity and expensive and time consuming maintenance. Furthermore solar thermal methods are not applicable to all regions and/or climates in that the presence of dust may block sunlight and/or surface area for evaporation, thereby increasing the time required for evaporation. In addition, the evaporated water is lost to the environment in the case of a pond, and it is not available then as a supply of drinking water. Finally toxic compounds such as sulfur-based compounds may evaporate and be transferred to the environment resulting in EHS issues.
Electrodialysis methods are also known for treating the first byproduct streams of RO systems. For example, an integrated RO and electrodialysis system is known from EP 2 070 583 A2, and the method of U.S. Pat. No. 6,030,535 uses a combination of an electrodialysis unit and evaporator to treat the concentrated saline solution byproduct stream from an RO system. Electrodialysis methods suffer the disadvantage though of being sensitive to membrane fouling and scaling, and they also require large quantities of direct current and their electric fields are only capable of removing ionic components. In conclusion, it would be desirable to have a process and apparatus to reduce the volume of concentrated saline solution byproduct streams of RO systems that has reduced energy requirements without requiring large infrastructures and without being susceptible to membrane fouling and having reduced susceptibility to scaling.